Figure 1
Water is absorbed from the soil by the plant root and transported to all parts of the plant. This passage of water is called the transpiration stream. Several factors play a role in pushing water up the transpiration stream against the force of gravity: Root Pressure and Osmosis – Water penetrates root cells through osmosis, a force created by unequal concentrations of water inside the root cells and the surrounding soil. Osmosis continues to push water upward due to unequal osmotic pressure across the plant. Capillary action or capillarity is the ability of narrow tubes to draw liquid upwards against the force of gravity. The flow of water in a plant’s thin xylem tubes is influenced by two opposite forces: adhesive forces of water molecules to the surface of the tube walls and cohesive forces that attract water molecules to each other. When the adhesive forces between the water and the tube walls are stronger than the cohesive forces between the water molecules, a capillary action is generated. Transpiration Pull is the major force driving water circulation in plants and trees. Water evaporates through the stomata located in the leaves: This process is called Transpiration. As water evaporates from leaves, more water is pulled up from the root system by osmosis – a process that causes water to flow from an area where it is plentiful to an area where it is scarce. The gas carbon dioxide or CO2 is needed for photosynthesis. During the day, the stomata open allowing the plant to “breathe in” CO2. This also greatly increases the amount of water lost through evaporation. Ultimately, 90% of water taken in by plants is lost through transpiration.
In this experiment water rising through the xylem is observed in a celery stalk (Apium graveolens( as it absorbs water colored with methylene blue dye.
| Water Circulation in Shoots and Leaves of Terrestrial Plants | The extent of water loss in transpiration is followed by measuring water intake by a shoot inserted into a flask full of water. Evaporation of water from the leaves leads to suction of the water from the flask. The suction of the water continuously increases the volume of air in the flask, thus causing a reduction in pressure (in accordance with Boyle’s Law) that can be recorded by the Pressure Sensor.
einstein™Tablet with MiLAB or Android /IOS Tablet with MiLAB and einstein™LabMate Two Pressure Sensors (150 – 1150 mbar) Five 250 ml glass flasks Two rubber stoppers with 2 holes each Piece of modeling clay Two syringe extenders* Two three-way valves* Magnifying glass Plastic knife 20 cm ruler Celery stalk Shoot of a tree or bush (Nerium oleander works well) 1% methylene blue dye *contained in einstein™ Pressure Kit
1. 2. 3. 4. 5. 6.
7. 8. 9.
Pour 100 ml of 1% methylene blue solution into three 250 ml flasks. Number them: 1-3. Divide the celery stalk along its length into 3 strips. Each strip should include an equal number and type of leaves - preferably the younger inner leaves, and the stalk. Observe the cross section under a magnifying glass. Try to identify the xylem tubes (see Figure 1). Place one strip in each flask. Wait five minutes, and then take one stalk out of the solution. Wipe off excess color. Cut a 1 cm piece from the lower edge of the stalk. Observe the cross section under the magnifying glass to see if the xylem bundles are blue. Count the number of colored xylem bundles you observe in the section. Keep cutting off 1 cm pieces from the bottom of the celery stalk until you reach a section without colored bundles (see Figure 2). Wait 10 minutes; take out the second stalk. Cut cross sections and check them under the magnifying glass as performed in steps 6 and 7. Count the number of xylem bundles you observe in the section. After 20 minutes take out the third stalk and repeat steps 6 and 7.
| Water Circulation in Shoots and Leaves of Terrestrial Plants |
Figure 2
10. Prepare a table to display your results: Height (cm) No. of colored tubes in the section 11. Calculate the average height the color has reached in each stalk. 12. Multiply the height by the number of colored bundles. For example: a. In one bundle the height was 5 cm: 5 cm X one bundle = 5 b. In three bundles the height was 4 cm: 4 cm X three bundles = 12 c. In five bundles the height was 3 cm: 3 cm X five bundles = 15 d. Sum up the total height in all the sections: 32 cm e. Divide it by the total number of bundles:9 f. The average height reached by the water after 5 minutes was 3.5 cm. 13. Calculate the average rate of water rise in the three leaves in cm per minute.
1.
Launch MiLAB (
2. 3. 4.
Connect the Pressure Sensors to ports of the einstein™ Tablet or einstein™ LabMate. Assemble the equipment as illustrated in Figure 3. Perform the experiment in an aerated and well-lit room. If possible, place the experimental systems close to the window. Choose a shoot of a tree or a bush whose leaves have a large surface area (either many small leaves or several big leaves). The surface of the shoot should be smooth and cylindrical, to ensure a tight fit in the stopper. Fill the two 250 ml flasks with water. Make sure the flasks are filled with water leaving only a 1mm gap at the top.
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6.
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| Water Circulation in Shoots and Leaves of Terrestrial Plants | 7. 8. 9. 10. 11. 12. 13.
Insert the shoot of a tree or bush into the flask with the two-holed stopper until it almost touches the flask’s bottom. (Seal with clay or other material if necessary.) Close the second flask with the other stopper. Insert a syringe extender into both stoppers (Figure 4). Attach three-way valves to each of the syringe extenders. Make sure the flasks are completely sealed; you may need to use clay or other material to complete the seal. Connect a Pressure Sensor to each three way valve. In the Current Setup Summary window choose Full Setup and use the table below to set up the experiment. Make sure that only the Pressure Sensors are selected under Measurements.
Figure 3
| Water Circulation in Shoots and Leaves of Terrestrial Plants |
Figure 4
Program the sensors to log data according to the following setup: Pressure (150 – 1150 mbar) Rate:
Every 1 sec
Duration:
20000 sec
Checking the experimental setup: Before starting the experiment, make sure that the flasks are tightly sealed. For more details see Sealing. Performing the experiment: 1.
Make sure that the experiment begins with both flasks at atmospheric pressure. Turn the three-way valves to position A (see Sealing), and then return to position B. The pressure in both the flasks should now equal atmospheric pressure.
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Tap Run ( ) to begin recording data. Follow the pressure in the Graph window of MultiLab4. Allow the experiment to run at least 25 min.
5.
Save your data by tapping Save (
).
For more information on working with graphs see: Working with Graphs in MiLAB 1.
To calculate the Transpiration rate you’ll need to create a difference graph.
| Water Circulation in Shoots and Leaves of Terrestrial Plants | 2. 3.
Save the graph. Select the graph of Input 1 (the control flask) then select the lowest point of Input 2.
4.
Select Functions (
5.
)
a.
Select Subtract from the Functions dropdown menu
b.
In the G1 drop down menu select Pressure -1. In the G2 drop down menu select Pressure -2.
c.
In the Name edit box enter a name (e.g. Difference).
Apply a linear fit to the difference graph: a. Select the difference graph b. Select Linear fit from the Functions dropdown menu. The fit equation will be displayed below the x-axis. c. The slope of the fit line is the measured rate of water loss in the experiment
An example of the graphs obtained in this experiment is shown below:
Pressure (mbar)
Pressure in the control system
Pressure in the experimental system
Figure 4
1. 2. 3. 4. 5. 6. 7.
What is the control used in this experiment? Why is a control flask necessary in the experiment? What is the effect of light on the rate of transpiration during the experiment? Would you expect similar changes in the dark? What would be the effect of an increase in humidity on the rate of water suction? Explain your answer. Why is a shoot with a large surface area of leaves necessary for this experiment? What is the source of water loss in this experiment? What would be the effect of covering the underside of the leaves with Vaseline?
| Water Circulation in Shoots and Leaves of Terrestrial Plants |
1. 2. 3. 4. 5. 6.
Design an experiment to investigate the effect of light on the rate of water loss. Explore the effect of wind and humidity on the rate of water loss. Explore the effect of covering the leaves with Vaseline on the rate of water suction. Explore the effect of surface area of leaves on the rate of water loss: Use shoots of different sizes and numbers of leaves. Cut the shoot and compare the number of xylem bundles with what you observed in the celery stalk. Compare the evaporation rate in another plant. Choose a plant with a high rate of respiration, Ceratonia siliqua for example.
| Water Circulation in Shoots and Leaves of Terrestrial Plants |